6
Chapter
Technical Guidance for Specific
Treatment Measures
In this Chapter:
 Technical guidance for stormwater treatment measures commonly used in San
Mateo County
Technical guidance is provided for the stormwater treatment measures listed in Table 6-1.
Table 6-1: Treatment Measures for which Technical Guidance is Provided
Treatment Measures
Bioretention area, including bioretention swale
Flow-through planter box
Tree well filter
Vegetated buffer strip
Infiltration trench
Extended detention basin
Pervious paving
Turf block and permeable joint pavers
Green roof
Rainwater harvesting and use
Media filter
Section
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
The technical guidance in this chapter is intended help prepare permit application submittals
for your project. Municipalities will require you to prepare more specific drawings taking into
consideration project site conditions, materials, plumbing connections, etc., in your
application. This technical guidance was developed using best engineering judgment and
based on a review of various documents and guidance from Water Board staff as available.
We look forward to working with Water Board staff to continue improving this guidance.
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6.1 Bioretention Areas
Best uses
 Any type of development
 Drainage area up to 2 acres
 Landscape design element
Advantages
 Detains low flows
 Landscape feature
 Low maintenance
 Reliable once established
Figure 6-1. Bioretention Area.
Source: City of Brisbane
Limitations
 Not appropriate
where soil is unstable
 Requires irrigation
 Susceptible to clogging –
especially if installed prior to
construction site soil
stabilization.
Bioretention areas1, or “rain gardens,” are concave landscaped areas that function as soil
and plant-based filtration devices that remove pollutants through a variety of physical,
biological, and chemical treatment processes. Bioretention areas can be any shape,
including linear. Linear bioretention areas are sometimes referred to as bioretention swales.
Bioretention areas normally consist of the following layers, starting from the top: a surface
ponding area, a layer of mulch, planting soil and plants, and an underlying rock layer with an
underdrain that connects to the municipal storm drain system.
Bioretention areas are designed to distribute stormwater runoff evenly within the surface
ponding area. The water is temporarily stored in the ponding area and percolates through
the planting soil, which is engineered to have a high rate of infiltration. From there, the water
filters down into the underlying rock layer.
The rock layer of the bioretention area may be designed to either maximize infiltration or
prevent infiltration to the underlying soils. In bioretention areas that maximize infiltration, the
underdrain is raised 6 inches above the bottom of the rock layer, and there is no liner
between the rock layer or planting soil and the surrounding soils. Maximizing infiltration is
only allowed where conditions are suitable for infiltration – check with the geotechnical
engineer. Where infiltration is precluded, the bioretention area is fully lined with waterproof
material, and the underdrain is placed at the bottom of the rock layer.
Design and Sizing Guidelines
DRAINAGE AREA AND SETBACK REQUIREMENTS


Set back from structures 10’ or as required by structural or geotechnical engineer, or
local jurisdiction.
Area draining to the bioretention area does not exceed 2 acres.
1
A bioretention area that is unlined and has a raised underdrain in the underlying rock layer to promote infiltration may also be
called a “bioinfiltration area”.
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C.3 STORMWATER TECHNICAL GUIDANCE
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Area draining to the bioretention area shall not contain a significant source of soil
erosion, such as high velocity flows along slopes not stabilized with vegetation or
hardscape.
Areas immediately adjacent to bioretention area shall have slopes more than 0.5% for
pavement and more than 1% for vegetated areas.
Bioretention areas, including linear treatment measures, shall not be constructed in
slopes greater than 4%, unless constructed as a series of bioretention cells. Separate
bioretention cells by check dams up to 24 inches high and at least 25 feet apart. The
slope within cells shall not exceed 4%. Bioretention cells are not recommended if overall
slope exceeds 8%.
If treatment measure is designed to infiltrate stormwater to underlying soils, a 50-foot
setback is needed from septic system leach field.
TREATMENT DIMENSIONS AND SIZING









Bioretention area may be sized to 4% of the impervious surface area on the project site.
The area of impervious surface multiplied by 0.04 sizing factor will equal the footprint of
the bioretention area. Alternatively, bioretention sizing may be calculated using the flowbased treatment standard, or the combination flow- and volume-based treatment
standard described in Section 5.1 based on the flow entering the basin at the treatment
flow rate over the initial hours of the storm until the treatment volume is attained.
The bioretention area shall be sized to either:
 Percolate the design treatment flow using a rate of 5 inches per hour. No additional
allowance is provided for storage or for infiltration rates in excess of 5 inches per
hour; or,
 Store the 24-hour treatment volume based on inflow at the water treatment rate for
the initial hours of the storm and outflow by infiltration.
Where there is a positive surface overflow, bioretention areas shall have freeboard of at
least 0.2 feet to the lowest structural member versus the 100-year storm water level in
the bioretention area, unless local jurisdiction has other requirements.
Where the bioretention area is in a sump that depends on outflow through a catch
basin, the bioretention area shall have a freeboard of at least 0.5 feet to the lowest
building finished floor elevation (including garage and excluding crawl space) for
conditions with the outlet 50 percent clogged, unless local jurisdiction has other
requirements. Where the freeboard cannot be provided, emergency pump may be
allowed on a case-by-case basis.
Minimum 2 inches between the crest of the emergency outfall riser and elevation of the
surface area.
The elevation of the surface area may vary as needed to distribute stormwater flows
throughout the surface area.
Side slopes do not exceed 3:1; downstream slope for overflow shall not exceed 3:1.
Surface ponding depths should vary, with a maximum depth of 12 inches. If ponding
depths exceed 6 inches, landscape architect shall approve planting palette for desired
depth.
The inlet to the overflow catch basin shall be at least 6 inches above the low point of the
bioretention planting area.
INLETS TO TREATMENT MEASURE
Flow may enter the treatment measure (see example drawings in Section 5.13):
 As overland flow from landscaping (no special requirements)
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM


 As overland flow from pavement (cutoff wall required)
 Through a curb opening (minimum 18 inches)
 Through a curb drain
 With drop structure through a stepped manhole (refer to Figure 5-3 in Chapter 5)
 Through a bubble-up manhole or storm drain emitter
 Through roof leader or other conveyance from building roof
Where flows enter the biotreatment measure, allow a change in elevation of 4 to 6
inches between the paved surface and biotreatment soil elevation, so that vegetation or
mulch build-up does not obstruct flow.
Cobbles or rocks shall be installed to dissipate flow energy where runoff enters the
treatment measure.
VEGETATION







Plant species should be suitable to well-drained soil and occasional inundation. See
planting guidance in Appendix A.
Shrubs and small trees shall be placed to anchor the bioretention area cover.
Tree planting shall be as required by the municipality. If larger trees are selected, plant
them at the periphery of bioretention area.
Underdrain trench shall be offset at edge of tree planting zone, as needed, to maximize
distance between tree roots and underdrain.
Use integrated pest management (IPM) principles in the landscape design to help avoid
or minimize any use of synthetic pesticides and quick-release fertilizer. Check with the
local jurisdiction for any local policies regarding the use of pesticides and fertilizers.
Irrigation shall be provided to maintain plant life.
Trees and vegetation do not block inflow, create traffic or safety issues, or obstruct
utilities.
SOIL CONSIDERATIONS SPECIFIC TO BIORETENTION AREAS






Planting soil shall have a minimum percolation rate of 5 inches per hour and a
maximum percolation rate of 10 inches/hour. Soil guidance is provided in Appendix K.
Check with municipality for any additional requirements.
Bioretention areas shall have a minimum planting soil depth of 18 inches.
Provide 3-inch layer of mulch in areas between plantings.
An underdrain system is generally required. Depending on the infiltration rate of in situ
soils, the local jurisdiction may allow installation without an underdrain on a case-bycase basis.
Underdrain trench shall include a 12-inch thick layer of Caltrans Standard Section 681.025 permeable material Class 2, or similar municipality-approved material. A
minimum 4-inch diameter perforated pipe shall be placed within the backfill layer. To
help prevent clogging, two rows of perforation may be used.
If there is at least a 10-foot separation between the base of the underdrain and the
groundwater table, and geotechnical conditions allow, there shall be at least 6-inch
separation between the perforated pipe and the base of the trench to allow percolation.
SOIL CONSIDERATIONS FOR ALL BIOTREATMENT SYSTEMS


Filter fabric shall not be used in or around underdrain trench.
If there is less than 10 feet separation to the groundwater table, an impermeable fabric
shall be placed at the base of the underdrain and the perforated pipe shall be placed on
the impermeable fabric.
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C.3 STORMWATER TECHNICAL GUIDANCE
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The underdrain shall include a perforated pipe with cleanouts and connection to a storm
drain or discharge point. Clean-out shall consist of a vertical, rigid, non-perforated PVC
pipe, with a minimum diameter of 4 inches and a watertight cap fit flush with the ground,
or as required by municipality.
There shall be adequate fall from the underdrain to the storm drain or discharge point.
Beginning December 1, 2011, soils in the area of inundation within the facility shall meet
biotreatment soil specifications approved by the Regional Water Board (Appendix K),
which supersede other soil specifications. The minimum percolation rate for the
biotreatment soil is 5 inches per hour. The long-term desired maximum infiltration rate is
10 inches per hour, although initial infiltration rate may exceed this to allow for tendency
of infiltration rate to reduce over time.
CONSTRUCTION REQUIREMENTS FOR ALL BIOTREATMENT SYSTEMS



When excavating, avoid spreading fines of the soils on bottom and side slopes.
Remove any smeared soiled surfaces and provide a natural soil interface into which
water may percolate.
Minimize compaction of existing soils. Protect from construction traffic.
Protect the area from construction site runoff. Runoff from unstabilized areas shall be
diverted away from biotreatment facility.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES



A Maintenance Agreement shall be provided.
Maintenance Agreement shall state parties’ responsibility for maintenance and upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement. Maintenance
plan templates are in Appendix G.
Figure 6-2: Cross Section, Bioretention Area
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM
12” MIN OF CLASS II PERMEABLE ROCK PER
CALTRANS SPECIFICATIONS OR SIMILAR
MUNICIPALITY-APPROVED MATERIAL.
Figure 6-3: Cross Section, Bioretention Area (side view)
Figure 6-4: Check dam (plan view and profile) for installing a series of linear bioretention cells in sloped
area
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C.3 STORMWATER TECHNICAL GUIDANCE
OR SIMILAR MUNICIPALITYAPPROVED MATERIAL.
Figure 6-5: Cross section of bioretention area showing inlet from pavement.
12” MIN OF CLASS II PERMEABLE ROCK PER
CALTRANS SPECIFICATIONS OR SIMILAR
MUNICIPALITY-APPROVED MATERIAL.
Figure 6-6: Bioretention area in landscaping to treat runoff from rainwater leaders (Not to Scale)
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM
12” MIN OF CLASS II PERMEABLE ROCK PER
CALTRANS SPECIFICATIONS OR SIMILAR
MUNICIPALITY-APPROVED MATERIAL.
Figure 6-7: Cross section of lined bioretention area, for locations where infiltration is precluded.
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C.3 STORMWATER TECHNICAL GUIDANCE
6.2 Flow-Through Planter
Best uses
 Treating roof runoff
 Next to buildings
 Dense urban areas
 Locations where
infiltration is not desired
Advantages
 Can be adjacent to
structures
 Multi-use
 Versatile
 May be any shape
 Low maintenance
Figure 6-8: At-grade flow-through planter. Source: City of Emeryville
Limitations
 Requires sufficient head
 Careful selection of
plants
 Requires level installation
 Susceptible to clogging
Flow-through planters are designed to treat and detain runoff without allowing seepage into
the underlying soil. They can be used next to buildings and other locations where soil
moisture is a potential concern. Flow-through planters typically receive runoff via
downspouts leading from the roofs of adjacent buildings. However, flow-through planters
can also be set level with the ground and receive sheet flow. Pollutants are removed as the
runoff passes through the soil layer and is collected in an underlying layer of gravel or drain
rock. A perforated pipe underdrain must be directed to a storm drain or other discharge
point. An overflow inlet conveys flows that exceed the capacity of the planter.
TREATMENT DIMENSIONS AND SIZING







Flow-through planters may be designed with a 4% sizing factor (percentage of the
surface area of planter compared to the surface area of the tributary impervious area).
The area of impervious surface multiplied by 0.04 sizing factor will equal the footprint of
the flow-through planter. Alternatively, calculations may be performed using either the
hydraulic sizing criteria for flow-based treatment measures or the hydraulic sizing
criteria for combination flow- and volume-based treatment measures, included in
Section 5.1.
Install an overflow weir adequate to meet municipal drainage requirements.
Flow-through planters can be used adjacent to building and within set back area.
Flow-through planters can be used above or below grade.
Size overflow trap for building code design storm, set trap below top of planter box walls.
Planter wall set against building should be higher to avoid overflow against building.
Elevation of the surface area may vary as needed to distribute stormwater flows
throughout the surface area.
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM

Minimum 2 and up to12 inches of water surface storage between the planting surface
and crest of overflow weir.
VEGETATION




Plantings should be selected for viability in a well-drained soil. See planting guidance in
Appendix A.
Use integrated pest management (IPM) principles in the landscape design to help avoid
or minimize any use of synthetic pesticides and quick-release fertilizer. Check with the
local jurisdiction for any local policies regarding the use of pesticides and fertilizers.
Irrigation shall be provided, as needed, to maintain plant life.
Trees and vegetation do not block inflow, create traffic or safety issues, or obstruct
utilities.
INLETS TO TREATMENT MEASURE
Flow may enter the treatment measure (see example drawings in Section 5.13):
 As overland flow from landscaping (no special requirements)
 As overland flow from pavement (cutoff wall required)
 Through a curb opening (minimum 18 inches)
 Through a curb drain
 With drop structure through a stepped manhole (refer to Figure 5-3 in Chapter 5)
 Through a bubble-up manhole or storm drain emitter
 Through roof leader or other conveyance from building roof
 Where flows enter the biotreatment measure, allow a change in elevation of 4 to 6
inches between the paved surface and biotreatment soil elevation, so that vegetation or
mulch build-up does not obstruct flow.
 Splash blocks, cobbles or rocks shall be installed to dissipate flow energy where runoff
enters the treatment measure.
 For long linear planters, space inlets to planter at 10-foot intervals or install flow
spreader.
SOIL CONSIDERATIONS SPECIFIC TO FLOW THROUGH PLANTERS







Waterproofing shall be installed as required to protect adjacent building foundations.
If site conditions permit infiltration to underlying soils, waterproofing is not required.
An underdrain system is generally required for flow through planters. Depending on the
infiltration rate of in situ soils, the local jurisdiction may allow installation without an
underdrain on a case-by-case basis.
Underdrain trench shall include a 12-inch thick layer of Caltrans Standard Section 681.025 permeable material Class 2, or similar municipality-approved material. A
minimum 4-inch diameter perforated pipe shall be placed within the backfill layer. To
help prevent clogging, two rows of perforation may be used.
Planting soil shall have minimum percolation rate of 5 inches per hour and a maximum
long-term percolation rate of 10 inches per hour. Soil specifications are provided in
Appendix K. Check with municipality for additional requirements.
The biotreatment soil shall be at least 18 inches thick.
Provide 3-inch layer of mulch in areas between plantings.
SOIL CONSIDERATIONS FOR ALL BIOTREATMENT SYSTEMS

Beginning December 1, 2011, soils in the area of inundation within the facility shall meet
biotreatment soil specifications approved by the Regional Water Board (Appendix K),
which supersedes other soil specifications. The minimum percolation rate for the
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C.3 STORMWATER TECHNICAL GUIDANCE
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biotreatment soil is 5 inches per hour. The long-term desired maximum infiltration rate is
10 inches per hour, although initial infiltration rate may exceed this to allow for tendency
of infiltration rate to reduce over time.
Filter fabric shall not be used in or around underdrain trench.
The underdrain shall include a perforated pipe with cleanouts and connection to a storm
drain or discharge point. Clean-out shall consist of a vertical, rigid, non-perforated PVC
pipe, with a minimum diameter of 4 inches and a watertight cap fit flush with the ground.
There shall be adequate fall from the underdrain to the storm drain or discharge point.
CONSTRUCTION REQUIREMENTS FOR ALL BIOTREATMENT SYSTEMS



When excavating, avoid spreading fines of the soils on bottom and side slopes.
Remove any smeared soiled surfaces and provide a natural soil interface into which
water may percolate.
Minimize compaction of existing soils. Protect from construction traffic.
Protect the area from construction site runoff. Runoff from unstabilized areas shall be
diverted away from biotreatment facility.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES



A Maintenance Agreement shall be provided.
Maintenance Agreement shall state the parties’ responsibility for maintenance and
upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement. Maintenance
plan templates are in Appendix G.
Figure 6-9: Plan view of long, linear planter, with inlets to the planter distributed along its length at 10’ intervals.
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM
4”
8”
Figure 6-10: Plan view of planter designed to disperse flows adequately with only one inlet to planter
12” of CLASS II PERMEABLE ROCK PER
CALTRANS SPECIFICATIONS OR SIMILAR
MUNICIPALITY-APPROVED MATERIAL
Figure 6-11: Cross section A-A of flow-through planter, shows side view of underdrain (Not to Scale)
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C.3 STORMWATER TECHNICAL GUIDANCE
4”
8”
OPTIONAL PLANTING MOUND PARAMETERS:
OR SIMILAR MUNICIPALITY-APPROVED
MATERIAL
Figure 6-12: Cross section B-B of flow-through planter, shows cross section of underdrain
Figure 6-13: Above-grade planters. Source: City of Portland
Figure 6-14: Close-up of Flow Through Planter. (Source:
City of Portland)
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6.3 Tree Well Filter
Best Uses
 Limited space
 Parallel to
roadways
Advantages
 Aesthetic
 Small surface land
use
 Blends with the
landscape
Figure 6-15: Non-proprietary tree well filters in Fremont use bio-retention soils
with an infiltration rate of 5 to 10 inches per hour. Spacing the units closely
together provides a total tree well filter surface area that is 4 percent of the
impervious surface area from which stormwater runoff is treated.
Limitations
 Can clog without
maintenance
 High installation
cost
 Systems with very
high infiltration
rates are allowed
only in Special
Projects
beginning
December 2011
Tree filters consist of one or multiple chambered pre-cast concrete boxes or hoops with a
small tree or shrub planted in a filter bed filled with engineered media or other absorptive
filtering media. As stormwater flows into the chamber, large particles settle out on the mulch
layer, and then finer particles and other pollutants are removed as stormwater flows through
the filtering media. Underground, physical, chemical and biological processes work to
remove pollutants from stormwater runoff. Stormwater flows through a specially designed
filter media mixture that has a high rate of infiltration. The mixture immobilizes some
pollutants, which may be decomposed and volatilized, or incorporated into the biomass of
the tree filter system's micro/macro fauna and flora. Stormwater runoff flows through the
media and into an underdrain system at the bottom of the container, where the treated
water is discharged. Tree filters are similar in concept to bioretention areas in function and
applications, with the major distinction that a tree filter has been optimized for high
volume/flow treatment, therefore the size of treatment area is proportionally less. A tree filter
takes up little space and may be used on highly developed sites such as landscaped areas,
green space, parking lots and streetscapes. A tree filter is adaptable and may be used for
developments, in all soil conditions to meet stormwater treatment needs. Beginning
December 1, 2011, manufactured tree well filters, and other tree well filters with long-term
rates of infiltration that exceed 10 inches per hour, will be allowed only in Special Projects,
as described in Appendix J.
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C.3 STORMWATER TECHNICAL GUIDANCE
Design and Sizing Guidelines
 Flows in excess of the treatment flow rate shall bypass the tree filter to a downstream
inlet structure or other appropriate outfall.
 Tree filters cannot be placed in sump condition; therefore tree filters shall have flow
directed along a flow line of curb and gutter or other lateral structure. Do not direct flows
directly to a tree filter.
 If a proprietary tree filter is used, it shall be reviewed by the manufacturer before
installation.
 For proprietary tree filters, manufacturer will size the tree filter to the impervious surface
of a site. The manufacturer shall certify the ratio of impervious area to treatment area for
the project. For example, Filterra states that a tree filter of 6 x 6-feet can treat 0.25 acres
of impervious surface.
 Proprietary tree filters are available in multi-sized pre-cast concrete drop in boxes, Sizes
range from 4 x 6-feet up to 6 x 12-feet boxes.
INLETS TO TREATMENT MEASURE
Flow may enter the treatment measure (see example drawings in Section 5.13):
 As overland flow from landscaping (no special requirements)
 As overland flow from pavement (cutoff wall required)
 Through a curb opening (minimum 18 inches)
 Through a curb drain
 With drop structure through a stepped manhole (refer to Figure 5-3 in Chapter 5)
 Through a bubble-up manhole or storm drain emitter
 Through roof leader or other conveyance from building roof
 Where flows enter the biotreatment measure, allow a change in elevation of 4 to 6
inches between the paved surface and biotreatment soil elevation, so that vegetation or
mulch build-up does not obstruct flow.
 Cobbles or rocks shall be installed to dissipate flow energy where runoff enters the
treatment measure.
VEGETATION




Suitable plant species are identified in Appendix A planting guidance.
Use integrated pest management (IPM) principles in the landscape design to help avoid
or minimize any use of synthetic pesticides and quick-release fertilizer. Check with the
local jurisdiction for any local policies regarding the use of pesticides and fertilizers.
Irrigation shall be provided, as needed, to maintain plant life.
Trees and vegetation do not block inflow, create traffic or safety issues, or obstruct
utilities.
SOIL REQUIREMENTS SPECIFIC TO TREE WELL FILTERS




Filter media in tree well filter shall be specialized for expected site pollutant loads.
Beginning December 1, 2011, if the long-term infiltration rate of media exceeds 10
inches per hour, use of the tree well filter will not be allowed, except for Special Projects
(see Appendix J).
An underdrain system is required for tree well filters.
Underdrain trench shall include a 12-inch thick layer of Caltrans Standard Section 681.025 permeable material Class 2, or similar municipality-approved material. A
minimum 4-inch diameter perforated pipe shall be placed within the backfill layer. To
help prevent clogging, two rows of perforation may be used.
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM

If there is at least a 10-foot separation between the base of the underdrain and the
groundwater table, and other conditions allow infiltration, there shall be at least 6-inch
separation between the perforated pipe and the base of the trench to allow percolation.
SOIL CONSIDERATIONS FOR ALL BIOTREATMENT SYSTEMS





Filter fabric shall not be used in or around underdrain trench.
If there is less than 10 feet separation to the groundwater table, an impermeable fabric
shall be placed at the base of the underdrain and the perforated pipe shall be placed on
the impermeable fabric.
The underdrain shall include a perforated pipe with cleanouts and connection to a storm
drain or discharge point. Clean-out shall consist of a vertical, rigid, non-perforated PVC
pipe, with a minimum diameter of 4 inches and a watertight cap fit flush with the ground,
or as required by municipality.
There shall be adequate fall from the underdrain to the storm drain or discharge point.
Beginning December 1, 2011, soils in the area of inundation within the facility shall meet
biotreatment soil specifications approved by the Regional Water Board (Appendix K),
which supersede other soil specifications. The minimum percolation rate for the
biotreatment soil is 5 inches per hour. The long-term desired maximum infiltration rate is
10 inches per hour, although initial infiltration rate may exceed this to allow for tendency
of infiltration rate to reduce over time.
CONSTRUCTION REQUIREMENTS FOR ALL BIOTREATMENT SYSTEMS



When excavating, avoid spreading fines of the soils on bottom and side slopes.
Remove any smeared soiled surfaces and provide a natural soil interface into which
water may percolate.
Minimize compaction of existing soils. Protect from construction traffic.
Protect the area from construction site runoff. Runoff from unstabilized areas shall be
diverted away from biotreatment facility.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES


A Maintenance Agreement shall be provided.
Maintenance Agreement shall state the parties’ responsibility for maintenance and
upkeep. Prepare a maintenance plan and submit with Maintenance Agreement.
Maintenance plan templates are in Appendix G.
Figure 6-16: Non-proprietary Tree Filter with Overflow Bypass. Source: University of New
Hampshire Environmental Research Group, 2006
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Figure 6-17: Cut Away View. Source: Americast, 2006. The use of this photo is for general
information only, and is not an endorsement of this or any other proprietary stormwater treatment
device.
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6.4 Vegetated Buffer Strip
Best Uses
 Roadside shoulders
 Landscape buffer
Advantages
 Minimal
maintenance
 Reliable
 Aesthetic appeal
 Adjustable to suit
site
Figure 6-18: Roadside Vegetated Buffer Strip
Source: www.cabmphandbooks.com
Limitations
 No large drainage
areas
 Thick cover
necessary
 Large size
requirements
 Minimal detention
provided
Vegetated buffer strips (grassed buffer strips, filter strips, and grassed filters) are vegetated
surfaces that are designed to treat sheet flow from adjacent surfaces. Vegetated buffer
strips function by slowing runoff velocities and allowing sediment and other pollutants to
settle and by providing some infiltration into underlying soils. Vegetated buffer strips were
originally used as an agricultural treatment practice and have more recently evolved into an
urban practice. With proper design and maintenance, vegetated buffer strips can provide
relatively high pollutant removal. In addition, the public views them as landscaped amenities
and not as stormwater infrastructure.
Design and Sizing Guidelines
TREATMENT DIMENSIONS AND SIZING



Strip shall be sized as long as the site will reasonably allow. The width in the direction of
flow shall be at least:
 5 feet where the length of flow across an impervious surface is less than 10-feet in
the direction of flow.
 At least 50 percent of the length of flow across an impervious surface where the
length of flow across an impervious surface is between 10 and 30 feet in the
direction of flow.
 At least 15 feet where the length of flow across an impervious surface is between
30 feet and 60 feet in the direction of flow.
Level spreaders shall be used if the length of flow across an impervious surface is
greater than 60 feet in the direction of flow. The level spreader shall distribute flows over
a length that will provide equivalent discharge per linear foot of level spreader as if the
flow to the vegetated buffer strip was from a surface with 60-feet length in the direction
of flow.
Slopes should not exceed 1-foot Vertical to 4-foot Horizontal (1:4).
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

Strip shall be free of gullies or rills.
Planting soil will be to a minimum depth of at least 6 inches. Native soil may be used as
a planting soil if approved by the landscape architect.
Strip shall be free of gullies or rills.
VEGETATION

Either grass or a diverse selection of other low growing, drought tolerant, native
vegetation should be specified. Vegetation whose growing season corresponds to the
wet season is preferred. See planting guidance in Appendix A.
 Use integrated pest management (IPM) principles in the landscape design to help avoid
or minimize any use of synthetic pesticides and quick-release fertilizer. Check with the
local jurisdiction for any local policies regarding the use of pesticides and fertilizers.
 Irrigation shall be provided, as needed, to maintain plant life.
 Trees and vegetation do not block inflow, create traffic or safety issues, or obstruct
utilities.
INLETS



Flow may enter the treatment measure (see example drawings in Section 5.13):
 As overland flow from landscaping (no special requirements)
 As overland flow from pavement (cutoff wall required)
 Through a curb opening (minimum 18 inches)
 Through a curb drain
 With drop structure through stepped manhole (refer to Figure 5-3 in Chapter 5)
 Through a bubble-up manhole or storm drain emitter
 Through roof leader or other conveyance from building roof
Where flows enter the biotreatment measure, allow a change in elevation of 4 to 6
inches between the paved surface and biotreatment soil elevation, so that vegetation or
mulch build-up does not obstruct flow.
If runoff is piped or channeled to the strip, a level spreader must be installed to create
sheet flow.
SOIL CONSIDERATIONS SPECIFIC TO VEGETATED BUFFER STRIPS





Check with municipality for planting soil requirements. Except where other municipal
requirements apply, planting soil shall have a minimum percolation rate of 2 inches per
hour and a maximum percolation rate of 10 inches/hour. If native soils do not meet this
percolation requirement, import soil meeting the Countywide Program’s dewatering soil
guidelines shall be used in the area of inundation.
Planting soil will be to a minimum depth of at least 6 inches.
No underdrain trench is needed where native soils are Hydrologic Soil Group A or B.
When placed on native hydrologic soil group C and D soils, drainage must be provided
to allow gravity drainage of the treatment soils. This may consist of underdrain trenches
or other means to assure that the biotreatment soil is able to fully dewater after storm
event.
Provide 3-inch layer of mulch in areas between plantings.
SOIL CONSIDERATIONS FOR ALL BIOTREATMENT SYSTEMS

Underdrain trench shall include a 12-inch thick layer of Caltrans Standard Section 681.025 permeable material Class 2, or similar municipality-approved material.
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


If there is less than 10 feet separation to the groundwater table, an impermeable fabric
shall be placed at the base of the underdrain and the perforated pipe shall be placed on
the impermeable fabric.
The underdrain shall include a perforated pipe with cleanouts and connection to a storm
drain or discharge point. Clean-out shall consist of a vertical, rigid, non-perforated PVC
pipe, with a minimum diameter of 4 inches and a watertight cap fit flush with the ground.
There shall be adequate fall from the underdrain to the storm drain or discharge point.
Beginning December 1, 2011, soils in the area of inundation within the facility shall meet
biotreatment soil specifications approved by the Regional Water Board (Appendix K),
which supersede other soil specifications. The minimum percolation rate for the
biotreatment soil is 5 inches per hour. The long-term desired maximum infiltration rate is
10 inches per hour, although initial infiltration rate may exceed this to allow for tendency
of infiltration rate to reduce over time.
CONSTRUCTION REQUIREMENTS FOR ALL BIOTREATMENT SYSTEMS



When excavating, avoid spreading fines of the soils on bottom and side slopes.
Remove any smeared soiled surfaces and provide a natural soil interface into which
water may percolate.
Minimize compaction of existing soils. Protect from construction traffic.
Protect the area from construction site runoff. Runoff from unstabilized areas shall be
diverted away from biotreatment facility.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES
Flow
Flow

A Maintenance Agreement shall be provided.
Maintenance Agreement shall state the parties’ responsibility for maintenance and
upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement. Maintenance
plan templates are in Appendix G.
Flow


Figure 6-19: Plan View, Vegetated Buffer Strip
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C.3 STORMWATER TECHNICAL GUIDANCE
12”
4”
Figure 6-20: Profile View, Vegetated Buffer Strip
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6.5 Infiltration Trench
Best Uses
 Limited space
 Adjacent to roadways
 Landscape buffers
Advantages
 Increases groundwater
recharge
 Removes suspended solids
 Used with other BMPs
 No surface outfalls
Figure 6-21. Infiltration Trench. Source: CASQA, 2003
Limitations
 Susceptible to clogging; fails
with no maintenance
 No high water tables
 Infiltration rate of existing
soils must exceed 0.5 in/hr
 No steep slopes
 Drainage area less than 5
acres
Infiltration trenches are appropriate in areas with well-drained (Type A or B) native soils.
Project applicants may wish to consult with Mosquito Abatement District staff for guidance
regarding mosquito controls. An infiltration trench is a long, narrow, excavated trench
backfilled with a stone aggregate, and lined with a filter fabric. Runoff is stored in the void
space between the stones and infiltrates through the bottom and into the soil matrix.
Infiltration trenches perform well for removal of fine sediment and associated pollutants.
Pretreatment using buffer strips, swales, or detention basins is important for limiting amounts
of coarse sediment entering the trench, which can clog and render the trench ineffective.
Infiltration practices, such as infiltration trenches, remove suspended solids, particulate
pollutants, coliform bacteria, organics, and some soluble forms of metals and nutrients from
stormwater runoff. The infiltration trench treats the design volume of runoff either
underground or at grade. Pollutants are filtered out of the runoff as it infiltrates the
surrounding soils. Infiltration trenches also provide groundwater recharge and preserve
base flow in nearby streams.
Design and Sizing Guidelines
DRAINAGE AREA AND SETBACK CONSIDERATIONS



When the drainage area exceeds 5 acres, other treatment measures shall be
considered.
Infiltration trenches work best when the upgradient drainage area slope is less than 5
percent. The downgradient slope shall be no greater than 20 percent to minimize slope
failure and seepage.
In-situ/undisturbed soils shall have a low silt and clay content and have percolation rates
greater than 0.5 inches per hour. In-situ testing is required to confirm percolation rate of
trench site. CASQA’s BMP Handbook recommends against using infiltration trenches
in Type C or D soils.
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C.3 STORMWATER TECHNICAL GUIDANCE



There shall be at least 10 feet between the bottom of the trench and the groundwater
table to prevent potential groundwater problems.
Trenches shall also be located at least 100 feet upgradient from water supply wells.
A setback of 100 feet from building foundations is recommended, unless a smaller
setback is approved by geotechnical engineer and local standard.
TREATMENT DIMENSIONS AND SIZING







The infiltration trench shall be sized to store the full 48-hour water quality volume.
A site-specific trench depth can be calculated based on the soil infiltration rate,
aggregate void space, and the trench storage time. The stone aggregate used in the
trench is normally 1.5 to 2.5 inches in diameter, which provides a void space of 35 to 40
percent. A minimum drainage time of 6 hours shall be provided to ensure satisfactory
pollutant removal in the infiltration trench. Trenches may be designed to provide
temporary storage of storm water. Trench depths are usually between 3 and 8 feet,
with a depth of 8 feet most commonly used.
The trench surface may consist of stone or vegetation (contact local municipality to
determine if vegetation is allowed) with inlets to evenly distribute the runoff entering the
trench. Runoff can be captured by depressing the trench surface or by placing a berm at
the down gradient side of the trench. The basic infiltration trench design utilizes stone
aggregate in the top of the trench to promote filtration; however, this design can be
modified by substituting pea gravel for stone aggregate in the top 1-foot of the trench.
Typically, there is about 35 to 40% void space within the rock.
Use trench rock that is 1.5 to 2.5 inches in diameter or pea gravel to improve sediment
filtering and maximize the pollutant removal in the top 1 foot of the trench.
Place permeable filter fabric around the walls and bottom of the trench and 1 foot below
the trench surface. The filter fabric shall overlap each side of the trench in order to cover
the top of the stone aggregate layer. The filter fabric prevents sediment in the runoff and
soil particles from the sides of the trench from clogging the aggregate. Filter fabric that is
placed 1 foot below the trench surface will maximize pollutant removal within the top
layer of the trench and decrease the pollutant loading to the trench bottom, reducing
frequency of maintenance.
The infiltration trench shall drain within 5 days to avoid vector generation.
An observation well is recommended to monitor water levels in the trench. The well can
be 4 to 6-inch diameter PVC pipe, which is anchored vertically to a foot plate at the
bottom of the trench.
INLET TO THE TREATMENT MEASURE


A vegetated buffer strip at least 5-feet wide, swale or detention basin shall be
established adjacent to the infiltration trench to capture large sediment particles in the
runoff before runoff enters the trench. If a buffer strip or swale is used, installation should
occur immediately after trench construction using sod instead of hydroseeding. The
buffer strip shall be graded with a slope between 0.5 and 15 percent so that runoff
enters the trench as sheet flow. The vegetated buffer strip or detention basin shall be
sized according to Sections 6.4 and 6.6 respectively.
If runoff is piped or channeled to the trench, a level spreader shall be installed to create
sheet flow.
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IF VEGETATION IS ALLOWED AT TRENCH SURFACE





Infiltration trenches can be modified by adding a layer of organic material (peat) or loam
to the trench subsoil. This modification enhances the removal of metals and nutrients
through adsorption. The modified trenches are then covered with a permeable
geotextile membrane overlain with topsoil and grass or stones.
If surface landscaping of the trench is desired, contact local municipality to determine if
this is allowed.
Plant species should be suitable to well-drained soil. See planting guidance in Appendix
A.
Use integrated pest management (IPM) principles in the landscape design to help avoid
or minimize any use of synthetic pesticides and quick-release fertilizer. Check with the
local jurisdiction for any local policies regarding the use of pesticides and fertilizers.
Irrigation shall be provided as needed to maintain plant life.
CON STRUCTION REQUIREMENTS



The drainage area must be fully developed and stabilized with vegetation before
constructing an infiltration trench. High sediment loads from unstabilized areas will
quickly clog the infiltration trench. During project construction, runoff from unstabilized
areas shall be diverted away from the trench into a sedimentation control BMP until
vegetation is established.
When excavating, avoid spreading fines of the soils on bottom and sides. Remove any
smeared soiled surfaces and provide a natural soil interface into which water may
percolate.
Minimize compaction of existing soils. Protect from construction traffic.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES



A Maintenance Agreement shall be provided.
Maintenance Agreement shall state the parties’ responsibility for maintenance and
upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement. Maintenance
plan templates are in Appendix G.
Figure 6-22: Infiltration trench cut-away view
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C.3 STORMWATER TECHNICAL GUIDANCE
Figure 6-23: Cutaway view: Infiltration Trench with Observation Well
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6.6 Extended Detention Basin
Best uses
 Detain low flows
 Can be expanded
to detain peak
flows
 Sedimentation of
suspended solids
 Sites larger than 5
acres
Advantages
 Easy to operate
 Inexpensive to
construct
 Treatment of
particulates
 Low maintenance
Figure 6-24: Extended Detention Basin. Photograph courtesy of Bill
Southard (DES Architects and Engineers)
Limitations
 Storage area
available
 Moderate
pollutant removal
Extended detention ponds (a.k.a. dry ponds, dry extended detention basins, detention
ponds, extended detention ponds) are basins whose outlets have been designed to detain
the stormwater runoff from a water quality design storm for some minimum time (e.g., 48
hours) to allow particles and associated pollutants to settle. Unlike wet ponds, these facilities
do not have a permanent pool. They can also be used to provide flood control by including
additional flood detention storage above the treatment storage area.
Beginning December 1, 2011, projects will no longer be allowed to meet stormwater
treatment requirements with stand-alone extended detention basins that are designed to
treat stormwater through the settling of pollutants and gradual release of detained
stormwater through an orifice. However, this type of extended detention basin could be
used as part of a treatment train, in which the basin stores a large volume of water, which is
gradually released to a bioretention area that meets the new MRP requirements for
biotreatment soils and surface loading area.
Design and Sizing Guidelines
TREATMENT DIMENSIONS AND SIZING




Extended detention basins shall be sized to capture the required water quality volume
over a 48-hour period. At least 10 percent additional storage shall be provided to
account for storage lost to deposited sediment.
Extended detention basin shall have no greater than 3:1 side slopes.
The optimal basin depth is between 2 and 5 feet.
A safety bench shall be added to the perimeter of the basin wall for maintenance when
basin is full.
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C.3 STORMWATER TECHNICAL GUIDANCE
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



Extended detention basin shall empty within five days of the end of a 6-hour, 100-year
storm event to avoid vector generation.
A 12-foot wide maintenance ramp leading to the bottom of the basin and a 12-foot wide
perimeter access road shall be provided. If not paved, the ramp shall have a maximum
slope of 5 percent. If paved, the ramp may slope 12 percent.
The extended detention basin shall have a length to width ratio of at least 1.5:1.
A fixed vertical sediment depth marker shall be installed in the sedimentation forebay.
The depth marker shall have a marking showing the depth where sediment removal is
required. The marking shall be at a depth where the remaining storage equals the
design water quality volume.
The detention basin is a volume-based treatment measure and requires detention time
to be effective. The basin shall not empty more than 50% of its treatment volume in less
than 24 hours to ensure treatment of runoff.
INLETS TO TREATMENT MEASURE



The inlet pipe shall have at least 1 foot of clearance to the basin bottom.
Piping into the extended detention basin shall have erosion protection. As a minimum, a
forebay with a 6-inch thick layer of Caltrans Section 72, Class 2 rock slope protection
shall be placed at and below the inlet to the extent necessary for erosion protection.
Check with municipality regarding trash screen requirements. Trash screen installation
may be required upstream of the pipe conveying water into the pond, in order to capture
litter and trash in a central location where it can be kept out of the pond until it is
removed.
OUTLETS AND ORIFICES

The outlet shall be sized with a drawdown time of 48 hours for the design water quality
volume. The outlet shall have two orifices at the same elevation sized using the
following equation:
-5
.5
a = (7x10 ) * A * (H-Ho) / CT
Where:
a = area of each orifice in square feet
A = surface area of basin at mid-treatment storage elevation (square feet)
H = elevation of basin when filled by water treatment volume (feet)
Ho = final elevation of basin when empty (bottom of lowest orifice) (feet)
C = orifice coefficient (0.6 typical for drilled orifice)
T = drawdown time of full basin (hours)
(Caltrans Method, Appendix B, Stormwater Quality Handbook, September 2002)

The orifices shall each be a minimum diameter of 1 inch. Extended detention basins are
not practical for small drainage areas because the minimum orifice diameter cannot be
met.
Each orifice shall be protected from clogging using a screen with a minimum surface
area of 50 times the surface area of the openings to a height of at least 6 times the
diameter. The screen shall protect the orifice openings from runoff on all exposed sides.
For each outlet, documentation shall be provided regarding adequacy of outlet
protection, and a larger stone size may be necessary depending on the slope and the
diameter of the outfall.


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VEGETATION




Plant species should be adapted to periods of inundation. See planting guidance in
Appendix A.
Use integrated pest management (IPM) principles in the landscape design to help avoid
or minimize any use of synthetic pesticides and quick-release fertilizer. Check with the
local jurisdiction for any local policies regarding the use of pesticides and fertilizers.
Irrigation shall be provided as needed to maintain plant life.
If vegetation is not established by October 1st, sod shall be placed over loose soils.
Above the area of inundation, a 1-year biodegradable loose weave geofabric may be
used in place of sod.
SOIL CONSIDERATIONS




If the groundwater level is within 10 feet of the ground surface, a liner shall be provided.
Beginning December 1, 2011, if the extended detention basin is designed to meet
biotreatment requirements, soils in the area of inundation within the facility shall meet
biotreatment soil specifications approved by the Regional Water Board (see Appendix
K), The minimum percolation rate for the biotreatment soil is 5 inches per hour. Longterm desired maximum infiltration rate is 10 inches per hour, although initial infiltration
rate may exceed this to allow for tendency of infiltration rate to reduce over time.
Beginning December 1, 2011, if extended detention basin is designed per biotreatment
requirements, the surface area shall be no smaller than what is required to
accommodate a 5” per hour stormwater runoff surface loading rate. A combination flow
and volume design basis, described in Section 5.1, may be used.
Beginning December 1, 2011, if the extended detention basin is NOT designed to meet
biotreatment requirements, it cannot function as a stand-alone treatment measure and
may only be used as part of a treatment train, followed by a biotreatment measure.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES



A Maintenance Agreement shall be provided.
Maintenance Agreement shall state the parties’ responsibility for maintenance and
upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement. Maintenance
plan templates are in Appendix G.
Figure 6-25. Side View of Riser
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C.3 STORMWATER TECHNICAL GUIDANCE
RISER STRAP
Figure 6-26. Top View of Riser (Square Design)
Figure 6-27. Plan View, Typical Extended Detention Basin
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6.7 Pervious Paving
Best uses



Parking areas
Common areas
Pathways
Advantages



Flow attenuation
Removes fine
particulates
Reduces need for
treatment
Limitations


Figure 6-28: The City of Menlo Park used permeable concrete for
parking stalls and standard paving in the drive aisles in this public
parking lot.

May clog without
periodic cleaning
Use in lightly
trafficked areas only
Higher installation
costs
Pervious paving is used for areas with light vehicle loading and lightly trafficked areas, such
as automobile parking areas. Table 6-2 shows possible applications for different types of
pervious paving. The term pervious paving describes a system comprised of a load-bearing,
durable surface together with an underlying layered structure that temporarily stores water
prior to infiltration or drainage to a controlled outlet. The surface is porous such that water
infiltrates across the entire surface of the material (e.g., crushed aggregate, porous concrete
and porous asphalt). If an area of pervious paving is underlain with pervious soil or pervious
storage material, such as a gravel layer sufficient to hold at least the Municipal Stormwater
Regional Permit Provision C.3.d volume of rainfall runoff, it is not considered an impervious
surface and can function as a self-treating area, as described in Section 4. 2. Please note
that projects that the CalGREEN Building Code does not define pervious paving in the
same way as the MRP. Projects that include pervious paving per CalGREEN requirements
must also verify that the pervious paving meets the MRP definition of pervious pavement.
Table 6-2:
Types of Pervious Paving and Possible Applications
Paver Type
Description
Possible Applications
Porous Asphalt
Open-graded asphalt concrete over an opengraded aggregate base, over a draining soil.
Contains very little fine aggregate (dust or sand)
and is comprised almost entirely of stone
aggregate and asphalt binder; surface void
content of 12-20%.
Low traffic use, such as parking
lots, travel lanes, parking stalls.
Surface may be too rough for
bicycle path.
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S T O R M W A T E R
T E C H N I C A L
G U I D A N C E
Table 6-2:
Types of Pervious Paving and Possible Applications
Paver Type
Description
Possible Applications
Pervious Concrete
A discontinuous mixture of coarse aggregate,
hydraulic cement and other cementitious
materials, admixtures, and water which has a
surface void content of 15-25% allowing water to
pass through.
Sidewalks and patios, low traffic
volume and low speed (less than
30 mph limit) bikeways, streets,
travel lanes, parking stalls, and
residential driveways.
Source: Design Guidelines for Permeable Pavements, Redwood City
Design and Sizing Guidelines
The design of each layer of the pavement must be determined by the likely traffic loadings
and the layer’s required operational life. To provide satisfactory performance, the following
criteria shall be considered.
SUBGRADE AND SITE REQUIREMENTS

The sub-grade shall be able to sustain traffic loading without excessive deformation.

The sub-grade shall be either ungraded in-situ material with a percolation rate of 5inches per hour, backfilled with coarser fill material, or installed with an underdrain that
will remove detained flows within the pervious paving and base.
Depth to groundwater shall be at least 10 feet from bottom of base.
Permeable pavements must be laid on a relatively flat slope, generally 5% or flatter. If
permeable pavements are laid on steep slopes, the open graded crushed aggregate
base may tend to migrate downhill, causing the surface to deform.


BASE LAYER







The granular capping and base layers shall give sufficient load-bearing to provide an
adequate construction platform and base for the overlying pavement layers.
The base aggregate particles shall be selected based on strength and durability when
saturated and subjected to wetting and drying.
To allow for subsurface water storage, the base must be open graded, crushed stone
(not pea gravel), meaning that the particles are of a limited size range, with no fines, so
that small particles do not choke the voids between large particles.
If the base layer is sized to hold at least the Municipal Stormwater Regional Permit
Provision C.3.d volume of rainfall runoff, the area of pervious paving is not considered
an impervious surface and can function as a self-treating area (see Section 4. 2).
If the base layer has sufficient capacity in the void space to store the C.3.d amount of
runoff for both the area of pervious paving and the area that drains to it, it is not
considered an impervious surface and can function as a self-retaining area, described in
Section 4.2.
If an underdrain is used, allow a minimum of 2 inches between underdrain and bottom
of base course. To be considered a self-treating area or self-retaining area, the
underdrain shall be positioned above the portion of the base layer that is sized to meet
the C.3.d sizing criteria.
Design calculations for the base shall quantify the following:
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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM




Type of soil, type of fill if used, permeability of base, k-values (psi/cubic inch)
Compressibility (clay and silt contents, organics, muck)
Traffic loading (in 18,000 lb. single axle loads)
Drainage routing of detained flows within the pervious pavement and base
(infiltration through minimum 5-inch per hour base into in-situ soils, or collection in
underdrain if percolation rate cannot be met with in-situ soils)
PAVEMENT MATERIALS


The pavement materials shall not crack or suffer excessive rutting under the influence of
traffic. This is controlled by the horizontal tensile stress at the base of these layers.
Pervious pavements require a single size grading to give open voids. The choice of
materials is therefore a compromise between stiffness, permeability and storage
capacity.
DESIGN AND INSTALLATION


Design shall be reviewed by manufacturer or National Ready Mixed Concrete
Association (NRMCA, www.nrmca.org).
Installation shall be by contractors familiar with pervious paving installation. Only
contractors with certification from NRMCA should be considered. More information can
be found at www.concreteparking.org.
Impervious aisle
Permeable stalls
Figure 6-29: Surface view of parking lot with pervious paving in lightly-trafficked areas. (Source: Bay
Area Stormwater Management Agencies Association [BASMAA], Start at the Source, 1999)
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T E C H N I C A L
G U I D A N C E
Figure 6-30: Pervious Concrete Installation. (Source: BASMAA, 1999). Depth of pervious concrete will vary
with type of usage.
Figure 6-31: Porous Asphalt Installation (Source: BASMAA, 1999)
Maintenance
A maintenance plan shall be provided.
Standards for Ongoing Maintenance and Upkeep:
 Keep landscaped areas well maintained.
 Prevent soil from washing onto the pavement. Pervious pavement surface shall be
vacuum cleaned using commercially available sweeping machines at following times:
 End of winter (April)
 Mid-summer (July / August)
 After autumn leaf-fall (November)
 Inspect outlets yearly, preferably before wet season. Remove accumulated trash/debris.
 When vacuum cleaning, inspect pervious paving for any signs of hydraulic failure.
As needed maintenance:
 If routine cleaning does not restore infiltration rates, then reconstruction of part of the
pervious surface may be required.
 The surface area affected by hydraulic failure should be lifted, if possible, for inspection
of the internal materials to identify the location and extent of blockage.
 Lift and replace surface materials as needed to restore infiltration.
Geotextiles may need complete replacement.
 Sub-surface layers may need cleaning and replacing.
 Removed silts may need to be disposed of as controlled waste.
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6.8 Turf Block and Permeable Joint Pavers
Best Uses
 Parking areas
 Common areas
 Lawn/landscape buffers
 Pathways
Advantages
 Flow attenuation
 Removes fine particulates
 Reduces need for treatment
Limitations
 May clog without periodic
cleaning
 Weeds
 Lightly-trafficked areas only
Figure 6-32: Turf Block and Pave Mat (Source: Georgia Stormwater  Higher installation costs
Handbook)
Turf block and permeable joint pavers are used for areas with light vehicle loading, such as
driveways, low-volume streets, street shoulders, and parking stalls (Table 6-3). The terms
turf block and permeable joint pavers describe systems comprised of a load-bearing,
durable surface together with a pervious soil that temporarily stores water, with overflow
conveyed to an outlet. The turf block surface is constructed of impermeable blocks
separated by spaces and joints, filled with soil and planted with turf, through which the water
can drain. Alternately, the spaces and joints of turf block may be filled with gravel.
Permeable joint pavers may be impermeable bricks, cobbles, natural stone, or modular unit
concrete pavers with permeable joints to allow runoff to percolate to subsurface layers.
Some pavers are designed with notched corners (Figure 6-37) to facilitate infiltration.
Where soil permeability is low, an underdrain system connected to the storm drain system
may be needed. Areas of turf block may be considered “self-treating areas,” and may drain
directly to the storm drain system if they do not receive runoff from impervious areas, as
allowed by the municipality. If an area of permeable joint pavers is underlain with pervious
soil or pervious storage material, such as a gravel layer sufficient to hold at least the
Municipal Stormwater Regional Permit Provision C.3.d volume of rainfall runoff, it is not
considered an impervious surface and can function as a self-treating area, as described in
Section 4. 2. Please note that projects that the CalGREEN Building Code does not define
pervious paving in the same way as the MRP. Projects that include permeable joint pavers
per CalGREEN requirements must also verify that the pavers meet the MRP definition of
pervious pavement.
Table 6-3:
Permeable Joint Paver Types and Possible Applications
Type
Description
Possible Applications
Brick
Solid unit paver laid on a permeable base
with sand joints.
Driveways, walkways, patios, public
sidewalks, plazas, low volume streets
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Table 6-3:
Permeable Joint Paver Types and Possible Applications
Type
Description
Possible Applications
Natural
Stone
Laid on pervious surface area in random
pattern with wide sand, gravel, or soil joints
(from 1/2 to 4 inches).
Driveways, walkways, patios, sidewalks,
plazas, low-use parking stalls
Turf
Blocks
Open celled unit paver filled with soil and
planted with turf. Sometimes the cells are
filled with crushed rock only.
Areas of low flow traffic and infrequent
parking, residential driveways and overflow
parking areas, emergency access roads,
utility roads, street shoulders, and outer
edges of commercial and retail parking lots
where low-use spaces are located.
Unit
Pavers
Discrete units set in a pattern on a prepared
base. Typically made of precast concrete in
shapes that form interlocking patterns, some
unit paver shapes form patterns that include
an open cell to increase permeability. Solid
unit pavers are made of impermeable
materials, but can be spaced to expose a
permeable joint set on a permeable base.
Parking stalls, private driveways,
walkways, patios, low volume streets, and
travel lanes, and bikeways.
Source: Design Guidelines for Permeable Pavements, Redwood City
Design and Sizing Guidelines
The design of each layer of the pavement must be determined by the likely traffic loadings
and their required operational life. To provide satisfactory performance, the following criteria
shall be considered:
 The subgrade shall be able to sustain traffic loading without excessive deformation.
 The turf block or permeable joint pavers shall give sufficient load-bearing to provide an
adequate support for loading.
 The paver materials should not crack or suffer excessive breakage under the influence
of traffic.
 Both turf block and pavers require a single size, grading base to provide open voids.
The choice of materials is thus a compromise between stiffness, permeability and
storage capacity.
 The uniformly graded single size material cannot be compacted and is liable to move
when construction traffic passes over it. This effect can be reduced by the use of
angular crushed rock material with a high surface friction.
 The base shall be sized for strength and durability of the aggregate particles when
saturated and subjected to wetting and drying. To allow for subsurface water storage,
the base must be open graded, crushed stone (not pea gravel), meaning that the
particles are of a limited size range, with no fines, so that small particles do not choke
the voids between large particles. If subsurface water storage is not an objective,
uncompacted soil with a sand bed to support the turf block or paver may be considered.
The base should be reviewed by manufacturer of turf blocks or pavers. Check with the
local jurisdiction regarding any local requirements for the base layer.
 If the base layer is sized to hold at least the Municipal Stormwater Regional Permit
Provision C.3.d volume of rainfall runoff, the area of pervious paving is not considered
an impervious surface and can function as a self-treating area (see Section 4. 2).
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

If the base layer has sufficient capacity in the void space to store the C.3.d amount of
runoff for both the area of pervious paving and the area that drains to it, it is not
considered an impervious surface and can function as a self-retaining area, described in
Section 4.3.
If an underdrain is used, allow a minimum of 2 inches between underdrain and bottom
of base course. To be considered a self-treating area or self-retaining area, the
underdrain shall be positioned above the portion of the base layer that is sized to meet
the C.3.d sizing criteria.
Figure 6-33: Profile of Brick Paver Installation (BASMAA, 1999)
Figure 6-34: Profile of Natural Stone Paver Installation (BASMAA, 1999)
Figure 6-35: Profile of Turf Block Installation (BASMAA, 1999)
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C.3 STORMWATER TECHNICAL GUIDANCE
Figure 6-36: Profile of Unit Paver Installation (BASMAA, 1999)
MAINTENANCE
A maintenance plan shall be provided.
Standards for Ongoing Maintenance and
Upkeep :
 Keep landscaped areas well
maintained
 The surface of the unplanted turf
block and permeable joint pavers
shall be vacuum cleaned using
commercially available sweeping
machines at the following times:
 End of winter (April)
 Mid-summer (July / August)
Figure 6-37: Unit Pavers, Redwood City
 After autumn leaf-fall (November)
 Planted turf block can be mowed, as needed.
 Inspect outlets yearly, preferably before the wet season. Remove trash and debris.
 When vacuum cleaning is conducted, inspect turf block and pavers for any signs of
hydraulic failure.
As needed maintenance:
 If routine cleaning does not restore infiltration
rates, reconstruct the part of pervious surface
that is not infiltrating.
 The surface area affected by hydraulic failure
should be lifted, if possible, for inspection of
the internal materials to identify the location
and extent of the blockage.
 Surface materials should be lifted and
replaced if damaged by brush (or abrasive)
Figure 6-38: Notched pavers (Source: Unigroupcleaning.
usa.org). Photo for example purposes only; it is
 Deposits may need to be disposed of as not an endorsement of any proprietary product.
controlled waste.
 Replace permeable joint materials as necessary.
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6.9 Green Roof
Best Uses
 For innovative
architecture
 Urban centers
Advantages
 Minimizes roof runoff
 Reduces “heat
island” effect
 Absorbs sound
 Provides bird habitat
 Longer “lifespan”
than conventional
roofs
Figure 6-39: Parking Lot with Turf-Covered Roof, Google building,
Mountain View
Limitations
 Sloped roofs require
steps
 Non-traditional
design
 High installation costs
A green roof can be either extensive, with a 3 to 7 inches of lightweight substrate and a few
types of low-profile, low-maintenance plants, or intensive with a thicker (8 to 48 inches)
substrate, more varied plantings, and a more garden-like appearance. The extensive
installation at the Gap Headquarters in San Bruno (Figure 6-39), has experienced relatively
few problems after nearly a decade in use. Native vegetation may be selected to provide
habitat for endangered species of butterflies, as at the extensive green roof of the Academy
of Sciences in San Francisco.
Design and Sizing Guidelines
 Green roofs are considered “self-treating areas” or “self-retaining areas” and may drain
directly to the storm drain, if they meet the following requirements specified in the MRP:
 The green roof system planting media shall be sufficiently deep to provide capacity
within the pore space of the media to capture 80 percent of the average annual
runoff.
 The planting media shall be sufficiently deep to support the long-term health of the
vegetation selected for the green roof, as specified by the landscape architect or
other knowledgeable professional.
 Design and installation is typically completed by an established vendor.
 Extensive green roof systems contain layers of protective materials to convey water
away from roof deck. Starting from the bottom up, a waterproof membrane is installed,
followed by a root barrier, a layer of insulation (optional), a drainage layer, a filter fabric
for fine soils, engineered growing medium or soil substrate, and plant material.
 The components of intensive green roofs are generally the same as those used in
extensive green roofs, with differences in depth and project-specific design application.
 Follow manufacturer recommendations for slope, treatment width, and maintenance.
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C.3 STORMWATER TECHNICAL GUIDANCE

Either grass or a diverse selection of other low growing, drought tolerant, native
vegetation should be specified. Vegetation whose growing season corresponds to the
wet season is preferred. See Appendix A for planting guidance.
 Green roof shall be free of gullies or rills.
 Irrigation is typically required.
 Beginning December 1, 2011, green roofs will need to meet green roof specifications (to
be included in Appendix L) approved by the Regional Water Board in order to be
considered biotreatment measures.
Maintenance
 Inspection required at least semiannually. Confirm adequate irrigation for plant health.
 Fertilize and replenish growing media as specified by landscape designer and as
needed for plant health. See Appendix A for alternatives to quick release fertilizers.
See www.greenroofs.com for information about and more examples of green roofs.
Figure 6-40: Extensive Green Roof at Gap Headquarters, San Bruno (William
McDonough & Partners)
Figure 6-42: Plants selected to support
endangered butterflies (California
Academy of Sciences, San Francisco)
Figure 6-41: Intensive Green Roof, Kaiser Center Parking
Garage, Oakland
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6.10 Rainwater Harvesting and Use
Best Uses
 High density residential or office
towers with high toilet flushing
demand.
 Park or low density development
with high irrigation demand.
 Industrial use with high nonpotable water demand.
Advantages
 Helps obtain LEED or other
credits for green building.
Limitations
 High installation and
maintenance costs.
 Low return on investment.
 Municipal permitting
requirements not standardized.
Figure 6-43: Rainwater is collected and used for
flushing toilets at Mills College, Oakland.
Rainwater harvesting systems area engineered to store a specified volume of water with no
discharge until this volume is exceeded. Storage facilities that can be used to harvest
rainwater include above-ground or below-ground cisterns, open storage reservoirs (e.g., ponds
and lakes), and various underground storage devices (tanks, vaults, pipes, arch spans, and
proprietary storage systems). Rooftop runoff is the stormwater most often collected in
harvesting/use system, because it often contains lower pollutant loads than surface runoff, and
it provides accessible locations for collection. Rainwater can also be stored under hardscape
elements, such as paths and walkways, by using structural plastic storage units, such as
RainTank, or other proprietary storage products. Water stored in this way can be used to
supplement onsite irrigation needs, typically requiring pumps to connect to the irrigation
system. Rain barrels are often used in residential installations, but typically collect only 55 to
120 gallons per barrel; whereas systems that are sized to meet Provision C.3 stormwater
treatment requirements typically require thousands of gallons of storage.
Uses of Harvested Water
Uses of captured water may potentially include irrigation, indoor non-potable use such as toilet
flushing, industrial processing, or other uses. As indicated in Appendix I, the Harvest and Use,
Infiltration and Evapotranspiration Feasibility/Infeasibility Criteria Report (Feasibility Report)
identified toilet flushing as the use that is most likely to generate sufficient demand to use the
C.3.d amount of runoff. The demand for indoor toilet flushing is most likely to equal to the C.3.d
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C . 3
S T O R M W A T E R
T E C H N I C A L
G U I D A N C E
amount of stormwater in high rise residential or office projects, and in schools. Irrigation
demand may equal the C.3.d amount of runoff in projects with a very high percentage of
landscaping.
System Components
Rainwater harvesting systems typically include several components: (1) methods to divert
stormwater runoff to the storage device, (2) an overflow for when the storage device is full, and
(3) a distribution system to get the water to where it is intended to be used. Filtration and
treatment systems are typically required for indoor uses of harvested rainwater (see Table 6-2).
LEAF SCREENS, FIRST-FLUSH DIVERTERS, AND ROOF WASHERS
These features may be installed to remove debris and dust from the captured rainwater before
it goes to the tank. The initial rainfall of any storm often picks up the most pollutants from dust,
bird droppings and other particles that accumulate on the roof surface between rain events.
Leaf screens remove larger debris, such as leaves, twigs, and blooms that fall on the roof. A
first-flush diverter routes the first flow of water from the catchment surface away from the
storage tank to remove accumulated smaller contaminants, such as dust, pollen, and bird and
rodent feces. A roof washer may be placed just ahead of the storage tank and filters small
debris for systems using drip irrigation. Roof washers consist of a tank, usually between 30and 50-gallon capacity, with leaf strainers and a filter.
TREATMENT METHODS
The Texas Manual on Rainwater Harvesting (3rd Edition, 2006) identifies two methods of
treatment used in rainwater harvesting systems for indoor use: chlorine and UV light. Chlorine
has a longer history of use in the US, and is still reported to be used by rainwater harvesters,
but it has drawbacks. Chlorine combines with decaying organic matter in water to form
trihalomethanes, a by-product that has been found to cause cancer in laboratory rats; some
users may find the taste and smell of chlorine objectionable; and chlorine does not kill Giardia
or Cryptosporidium, which are cysts protected by their outer shells. UV light has more
recently become common practice in U.S. utilities. Bacteria, virus, and cysts are killed by
exposure to UV light. The water must go through sediment filtration before the ultraviolet light
treatment because pathogens can be shadowed from the UV light by suspended particles in
the water. In water with very high bacterial counts, some bacteria will be shielded by the bodies
of other bacteria cells. UV lights are benign: they disinfect without leaving behind any
disinfection by-products, and they use minimal power for operation.
Table 6-2
Typical Water Quality Guidelines from the Texas Rainwater Harvesting Manual
Use
Non-potable
indoor uses
Outdoor uses
Minimum Water Quality
Guidelines
 Total coliforms < 500 cfu
per 100 mL
 Fecal coliforms < 100 cfu
per 100 mL
N/A
Suggested Treatment Guidance
 Pre-filtration – first flush diverter
 Cartridge filtration – 5 micron sediment filter
 Disinfection – chlorination with household
bleach or UV disinfection
 Pre-filtration – first flush diverter
Source: Low Impact Development Manual for Southern California, Low Impact Development Center,
2010, which, in turn, cites the Texas Rainwater Harvesting Manual for this information.
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Design and Sizing Guidelines
HYDRAULIC SIZING


If a rainwater harvesting system will be designed to meet Provision C.3 stormwater
requirements, there must be sufficient demand to use 80 percent of the average annual
rainfall runoff, as specified in Provision C.3.d.
If the project’s completed Rainwater Harvesting Worksheet (or other project-specific
calculation) indicates that there is sufficient demand, size the cistern (or other storage
device) to achieve the maximum drawdown time indicated in Table 9 of the Feasibility
Report (included in Appendix I).
DESIGN GUIDELINES FOR ALL SYSTEMS






Equip water storage facilities covers with tight seals, to reduce mosquito-breeding risk.
Follow mosquito control guidance in Appendix F.
Water storage systems in proximity to the building may be subject to approval by the
building official. The use of waterproofing as defined in the building code may be required
for some systems, and the municipality may require periodic inspection. Check with
municipal staff for the local jurisdiction’s requirements.
Do not install rainwater storage devices in locations where geotechnical/stability concerns,
such as a slope above 10%, may prohibit the storage of large quantities of water.
Provide separate piping without direct connection to potable water piping. Dedicated
piping should be color coded and labeled as harvested rainwater, not for consumption.
Faucets supplied with non-potable rainwater should include signage identifying the water
source as non-potable and not for consumption.\
The harvesting system must not be connected to the potable water system at any time.
When make-up water is provided to the harvest/reuse system from the municipal system,
prevent cross contamination by providing a backflow prevention assembly on the potable
water supply line, an air gap, or both, to prevent harvested water from entering the potable
supply. Contact local water system authorities to determine specific requirements.
DESIGN GUIDELINES FOR INDOOR USE


Avoid harvesting water for indoor use from roofs with architectural copper, which may
discolor porcelain.
Provide filtration of rainwater harvested for indoor non-potable use, as required by the
plumbing code and any municipality-specific requirements.
DESIGN GUIDELINES FOR IRRIGATION USE



Water diverted by a first flush diverter may be routed to a landscaped area large enough to
accommodate the volume, or a hydraulically-sized treatment measure.
First flush diverters shall be installed in such a way that they will be easily accessible for
regular maintenance.
Do not direct to food-producing gardens rainwater harvested from roofs with wood shingles
or shakes (due to the leaching of compounds), asphalt shingles, tar, lead, or other
materials that may adversely affect food for human consumption.
MAINTENANCE CONSIDERATIONS FOR ALL TREATMENT MEASURES


A Maintenance Agreement shall be provided and shall state the parties’ responsibility for
maintenance and upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement.
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6.11 Media Filter
Best Uses
 Limited space
 Underground
 Used following a
separation unit,
such as swirl
concentrator
Advantages
 Less area required
 Customized media
 Customized sizing
Figure 6-44. System C Filter Cartridge, Typically Used as
Part of Treatment Train. Source: CONTECH Stormwater
Solutions, 2006. (Note: The proprietary media filters shown are
for general information only and are not endorsed by the
Countywide Program. An equivalent filter may be used.)
Limitations
 No removal of trash
without pretreatment
 High installation and
maintenance costs.
 Media filtration will
be allowed only for
some “special
projects” beginning
December 2011
Stormwater media filters are usually two-chambered, including a pretreatment settling basin
and a filter bed filled with sand or other absorptive filtering media. As stormwater flows into
the first chamber, large particles settle out, and then finer particles and other pollutants are
removed as stormwater flows through the filtering media in the second chamber. There are
currently three types of manufactured stormwater media filter systems. Two are similar in
that they use cartridges of a standard size (filter types B and C, seen above). The cartridges
are placed in vaults; the number of cartridges are a function of the design flow rate. The
water flows laterally (horizontally) into the cartridge to a center well, then downward to an
underdrain system. The third product (type A) is a flatbed filter, similar in appearance to sand
filters.
Note: Beginning December 1, 2011, the use of media filters will not be allowed, except
as may be indicated in Special Projects criteria (Appendix J).
Design and Sizing Guidelines
There are currently three types of stormwater filter systems:
Filter System A:
 This system is similar in appearance to a slow-rate sand filter.
 The media is cellulose material treated to enhance its ability to remove hydrocarbons
and other organic compounds. The media depth is 12 inches.
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


Operates at a very high rate, at peak flows. Normal operating rates are much lower
assuming that the stormwater covers the entire bed at flows less than the peak rate.
System uses a swirl concentrator for pretreatment.
As the media is intended to remove sediments (with attached pollutants) and organic
compounds, it would not be expected to remove dissolved pollutants such as nutrients
and metals unless they are complexed with the organic compounds that are removed.
Filter System B:
 Uses a simple vertical filter consisting of 3-inch diameter, 30-inch high slotted plastic
pipe wrapped with fabric.
 The standard fabric has nominal openings of 10 microns. The stormwater flows into the
vertical filter pipes and out through an underdrain system. Several units are placed
vertically at 1-foot intervals to give the desired capacity.
 The filter bay has a typical emptying time of 12 to 24 hours.
 In a cartridge filter the media is fabric, therefore the system may not remove dissolved
pollutants. It does remove pollutants attached to the sediment that is removed.
Filter System C:
 The system uses vertical cartridges in which stormwater enters radially to a center well
within the filter unit, flowing downward to an underdrain system.
 Flow is controlled by a passive float valve system, which prevents water from passing
through the cartridge until the water level in the vault rises to the top of the cartridge.
 Full use of the entire filter surface area and the volume of the cartridge is assured by a
passive siphon mechanism as the water surface recedes below the top of the cartridge.
 A balance between hydrostatic forces assures a more or less equal flow potential
across the vertical face of the filter surface. The filter surface receives suspended solids
evenly in this system.
 Absent the float valve and siphon systems, the amount of water treated over time per
unit area in a vertical filter is not constant, decreasing with the filter height; furthermore, a
filter would clog unevenly.
 Restriction of the flow using orifices ensures consistent hydraulic conductivity of the
cartridge as a whole by allowing the orifice, rather than the media, whose hydraulic
conductivity decreases over time, to control flow.
 Manufacturers offer several media types used singly or in combination (dual- or multimedia). Total media thickness is about 7 inches. Some media, such as fabric and
perlite, remove only suspended solids (with attached pollutants). Media that also
remove dissolved pollutants include compost, zeolite, and iron-infused polymer.
Pretreatment occurs in an upstream unit and/or the vault within which the cartridges are
located. Water quality volume or flow rate (depending on the particular product) is
determined by local governments or sized so that 85% of the annual runoff volume is
treated.
All 3 types of media filter shall have a pretreatment system in place such as a swirl
concentrator.
MAINTENANCE

A Maintenance Agreement shall be provided.
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

Maintenance Agreement shall state the parties’ responsibility for maintenance and
upkeep.
Prepare a maintenance plan and submit with Maintenance Agreement. Maintenance
plan templates are in Appendix G.
Figure 6-45. Cut Away Profile Views, System A Filter
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Figure 6-46. Profile View, Typical System C Filter Array.
Source: CONTECH Stormwater Solutions, 2006. (Note: The
proprietary media filters shown are for general information
only and are not endorsed by Countywide Program.
Figure 6-47. Plan View, Typical System C Filter Array. Source: CONTECH Stormwater
Solutions, 2006. (Note: The proprietary media filters shown are for general information only
and are not endorsed by Countywide Program.
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